Method for encapsulation



April 11, 1967 Y e. FORMAN 3,313,034

METHOD FOR ENCAPSULATION Filed March 9, 1964 w wm W Patented Apr. 11,1967 3,313,084 METHOD FOR ENCAPSULATION Benjamin G. Forman, Massapequa,N.Y., assignor to Columbia Technical Corporation, Woodside, N.Y. FiledMar. 9, 1964, Ser. No. 350,333 1 Claim. (Cl. 53-22) This inventionrelates generally to the field of protective coverings, and moreparticularly to an improved means and method for encapsulatingelectronic components and assemblies against the deleterious effects ofmoisture, changes in temperature, changes in atmospheric pressure andother environmental stresses, and the like, and to provide additionalmechanical strength as protection against the effects of shock andvibration.

While it is known in the art to provide such protection by dipping thestructure to be protected into molten thermoplastic material, it ispreferable to avoid subjecting the protected components from thetemperature involved in using such a process. In addition, the coatingobtained is usually non-uniform in thickness, and sharp corners normallybecome too thinly coated, while pockets formed between adjacentelectronic components become too heavily coated, with a consequent lossin flexibility. It is also known in the art to spray a liquid coating inan evaporable vehicle, and allowing the vehicle to evaporate to leave aprotective coat upon the article being sprayed. However, such a processis time-consuming, and normally requires more than a few coats to buildup a substantial protective film. It is known in the art that polyestersand epoxies are used to encapsulate printed circuit assemblies. However,repairability is then impossible.

It is therefore among the principal objects of the present invention toprovide an improved means and method for the encapsulation of electroniccomponents and similar articles in which the above-mentioneddisadvantages have been substantially eliminated.

Another object of the invention lies in the provision of an improvedmeans and method for encapsulation which may require a minimum of timeand material in the use thereof.

A further object of the invention lies in the provision of an improvedmeans and method for encapsulation which will produce a protectivecovering of relatively constant thickness while yet conforming toirregularities in surface or shape of the article enclosed.

A further object of the invention lies in the provision of an improvedmeans and method for encapsulation in which the over-all cost thereofmay be of a reasonably low order, with consequent wide sale,distribution and use. In addition, the cost of repair of the item to beencapsulated would be substantially lower.

A feature of the invention lies in the fact that it may be employed inthe encapsulation of both planar and relatively non-planar articles.

Another feature of the invention lies in the fact that the electricalconnection of an encapsulated object with corresponding means disposedoutside of the encapsulating envelope is readily available after theencapsulation has been performed.

These objects and features, as well as other incidental ends andadvantages, will more fully appear in the progress of the followingdisclosure, and be pointed out in the appended claim.

In the drawing, to which reference will be made in the specification,similar reference characters have been employed to designatecorresponding parts throughout the several views.

FIGURE 1 is a plan view of a conventional printed circuit board elementhaving attached electronic components mounted thereon prior toencapsulation.

FIGURE 2 is a side elevational view showing a first step in thedisclosed encapsulating method.

FIGURE 3 is a plan view showing a subsequent step in the encapsulatingprocedure.

FIGURE 4 is a schematic view showing a third step in the method.

FIGURE 5 is a schematic View showing a fourth step in the method.

FIGURE 6 is a schematic view showing a fifth and final step in themethod.

In accordance with the invention, there is disclosed in FIGURE 1 in thedrawing a printed circuit board element, generally indicated byreference character 10, of a type widely used in the electronicsindustries and related arts. The element 10 is selectivelyinterconnectable with similar elements through means of interlockingplugs or jacks 9 which abut a free edge of a base member 11. The basemember 11 is normally formed from resin-impregnated textile materials,glass cloth or paper to obtain high strength consistent With low weight,and mounts a plurality of electronic components 12 which are usuallyinterconnected with a printed circuit on the base member. Although thebase member may be of any desired planar or non-planar configuration, itis normally of rectangular shape, being bounded by a pair of longersides 13 and 14, shorter sides 15 and 16, as well as an upper surface 17and a lower surface 18. Thus, as vmay be seen in FIGURE 2, the device 10is of generally planar configuration, but includes a plurality of raisedshapes of irregular form depending upon the particular electroniccomponent mounted thereupon.

Referring to FIGURE 2 in the drawing, after the device 10 has beensuitably cleaned of any dust particles thereon, the same is subjected toa light coating over the entire area thereof with a thin layer of asuitable sealing synthetic resin. I have found polyamid and/ornitrocellulose may be rapidly sprayed to a thickness of one or twothousandths of an inch. In order to assure that all exposed areas,including the surfaces of the electronic components 12, are covered, thecoating is preferably app-lied by a spray gun or air brush 20 whichpermits relative movement between the nozzle and the device 10, wherebythe exposed surfaces may be sprayed at different angles. Where the resinis applied in a volatile vehicle, evaporation is almost instantaneous,and the subsequent step described below may he proceeded with almostimmediately. It is further possible to coat printed circuit boards withthe above-described resins before the assembly of the components. Theseresins are solderaole. This will allow for complete encapsulation of thecomponents without any adhesion to the components. This enables simplerepairs.

Referring to FIGURE 3, as a subsequent step there is provided a bag-likecontainer 23, which may be of cellulose acetate, or any otherthermoplastic material which is compatible with the previously appliedcellulose acetate spray coating. The bag 23 is preferably of flattenedcon figuration, of dimensions sufiiciently large to permit ingress ofthe device 10. In the preferred form, it includes first and secondlaminae 24 and 25, respectively, which are sealed along three sidesthereof to provide free edges 26 and 27, interconnected side edges 28and 29, and an interconnected end edge 30, into which there has beenincorporated a short length of metallic or synthetic resinous tubing 31,the outer surface of which is in hermetically sealed relation withrespect to the end edge 30. As seen in FIGURE 3, the device 10, afterbeing spray-coated, is conveniently inserted into the confines of thebag 23.

Referring to FIGURE 4, as a subsequent step the free edges 26 and 27 areheat-sealed together to provide a hermetically sealed enclosure, and thetubing 31 is connected to air exhaustion means 32. The now-closed edge35 resembles the edges 28-30.

213 The now-closed bag 23 is form of infrared lamps or other suitableheating means. As the bag 23 softens under the action of heat, to becomethermoplastic, the air exhaustion means 32 is placed in operation,resulting in the drawing together of the first and second laminae 24 and25 to closely conform to the precise configuration of the device 10.Where individual wires or conductors stand free from the surfaces of thebase member 11 or components 12, under sufficient vacuum the softenedmaterial will interconnect beneath the conductors to completely enclosethe same.

Referring to FIGURE 5, as a subsequent step, the nowsealed bag 23 may betrimmed to the external configuration of the base member 11, wherein thenow-encapsulated device is of no greater appreciable size than prior toencapsulation. As a last step, as seen in FIGURE 6, the encapsulatingmedium is removed from the jacks 9 to permit the making of properelectrical contact when the device 10 is installed.

The laminae are applied hot, and the outside pressure seals the laminaeto the board. Once the bag ".23 has sufiiciently cooled, it may behandled extensively without damage either to the bag 23 or thecomponents 12. Should, however, it become necessary to replace anindividual component 12, the same may be reached by merely cuttingthrough the desired first or second lamina 24-25 to the correspondingsurface 17-18 of the base member 11, and that individual electroniccomponent disposed directly therebeneath may be conveniently replaced.Normally, such an operation will not disturb the encapsulation of theremaining electronic components, and the newly installed component needmerely be re-covered using a suitable resin (not shown).

I Wish it to be understood that I do not consider the invention limitedto the precise details of structure shown and set forth in thisspecification, for obvious modifications will occur to those skilled inthe art to which the invention pertains.

subjected to the action of a plurality of heat sources 36 and 37, whichmay be in the I claim:

The method of encapsulating electric circuit boards, electronicassemblies, and similar articles of generally planar or non-planarconfiguration, said boards having a plurality of electronic devicesmounted upon at least one planar surfa-cethereof, comprising the stepsof:

(a) providing a thermoplastic bag of material compatible withsaidfirst-mentioned plastic material, said bag having a configurationcorresponding generally to that of said board, and having at least onefree edge defining an opening, said bag having tubing means extendingthrough said bag;

(b) coating the exposed surfaces of said board and devices with athermoplastic material, said coating material being compatible andheat-scalable with the thermoplastic material comprising said bag;

(c) inserting said circuit board through the opening defined by saidfree edge to be completely disposed within said bag;

(d) heating the remaining material comprising said bag to a plasticstate;

(e) exhausting the atmosphere within said bag, Whereby atmosphericpressure will cause the material of said bag to flow about saidelectronic elements and contact the coated surfaces of said boards toform and heat-seal a vacuum enclosure; and (f) severing those portionsof said bag extending outwardly of said board.

References Cited by the Examiner UNITED STATES PATENTS TRAVIS S.MCGEHEE, Primary Examiner.

